Published: December 15, 1987

That decision brings the official rate, now at 2,200 pesos to the dollar, more in line with the free-market rate of 2,350, at least for the moment.

A foreign diplomat said this afternoon that the package was likely to include wage and price increases as well as a Government pledge of no further rises for several months. He predicted that President Miguel de la Madrid would also announce a plan to reduce the country's rising deficit, which is running at a record 17.4 percent of the gross domestic product. A 'Coordinated' Program

The devaluation decision is expected to be followed almost immediately by other measures intended to halt the sudden and rapid decline of the economy. In a statement issued this afternoon, the Bank of Mexico said it would make public this evening ''a coordinated economic program between the Government and the worker, peasant and business sectors.''

But late tonight officials said talks between the Government and labor unions were continuing. No new date was given for an announcement of the full economic package, but officials said talks would be held on Tuesday.

In a separate communique later this evening, President de la Madrid's office said the devaluation would stimulate exports and discourage imports, thereby ''fortifying the balance of payments'' and ''bettering the competitive international position'' of Mexican manufacturers. The manufacturers have been hurt by a growing gap between the devaluation of the peso and the rate of inflation. Cut in Tariff on Imports

But to counteract anticipated increases in the price of imported goods, the Government announced that, effective immediately, it was reducing the maximum tariff on imports to 20 percent, from 40 percent, on ''a wide variety'' of products. Tariffs on selected consumer products would be slashed by 25 percent, and a 5 percent import tax would be suspended, the statement said.

No specific products were mentioned, but the Government statement made it clear that officials hoped the tariff measure would diminish the inflationary effect of the devaluation by allowing manufacturers to import raw materials and capital goods at a lower real cost, while continuing to give consumers access to foreign goods at reasonable prices.

On the controlled market, the peso has been changing at a much slower rate than on the free market. At the start of business this morning, it was trading at 1,805.2 to the dollar, more than 20 percent below the free-market rate. The controlled market rate applies to about 80 percent of Mexico's foreign exchange transactions, and both exports and imports are calculated at the official rate.

The devaluation came nearly four weeks after the Government ended its efforts to support the peso on the free market. The peso immediately went from 1,700 to 2,700 to the dollar, before stabilizing.

Since then, inflation has surged to a record 144 percent, and the country's principal labor federation has threatened a general strike unless it received an immediate 46 percent wage rise. Silent President Criticized

Mr. de la Madrid has been largely silent about the crisis, drawing criticism from business leaders, the political opposition and even some members of his own party.

Taken in isolation, the currency devaluation would make Mexico's exports cheaper, and therefore more competitive in the United States and elsewhere. At the same time, imports would be 22 percent more expensive.

A little more than two-thirds of Mexico's imports come from the United States, and 62 percent of Mexico's exports go to the American market. Mexican exports reached $17.7 billion in 1986, while imports from the United States were $12.5 billion, according to American Embassy statistics.

Mexico's exports of oil are unlikely to be affected by today's devluation, because they are denominated in dollars.

''It's difficult to sustain that large a gap,'' one Mexican official said this afternoon of the difference between the official and free-market rates. ''This will reduce the pressure.'' The official said today's announcement ''makes sense in the context of the whole package'' to be announced tonight. But he declined to disclose any details of the new Government program.

The gap between the controlled and free-market exchange rates has generated considerable pressure on Mexico's foreign reserves, estimated at about $15 billion. When the Government announced on Nov. 18 that it would no longer support the peso, it said it was taking that decision to prevent a flight of capital. $3 Billion in Capital Flight A Mexican economist said many exporters and importers who previously acquired dollars on the free market had in recent weeks shifted to the controlled market to acquire their dollars more cheaply. Capital flight over the last eight weeks has been estimated to be as much as $3 billion. 39.4% PERU DEVALUATION